1. Field of the Invention
The invention relates to radioactive therapeutic seeds. More particularly, the invention relates to improved radioactive therapeutic seeds for the treatment of oncological and other medical conditions.
2. State of the Art
Radioactive seed therapy is a well known and well accepted medical procedure for the treatment of various oncological and other medical conditions. Seed therapy, also known as brachytherapy typically involves the implantation of fifty to one hundred tiny capsules (seeds) into or around a treatment site. The capsules contain a radioactive isotope which irradiates the treatment site at close range without adversely affecting other parts of the body. Brachytherapy has been used successfully in the treatment of various types of cancers such as prostate cancer. It has also been used to prevent the growth or regrowth of tissues in the treatment of various occlusive diseases such as arteriosclerosis and arthrosclerosis subsequent to balloon angioplasty.
Radioactive therapeutic seeds are carefully designed to possess several important qualities. First, they must be relatively small, approximately 0.025 inch in diameter and approximately 0.16 inch long so that they may be implanted using minimally invasive instruments and techniques. Second, the radioactive isotope must be enclosed in a biocompatible protective package since the seeds are typically not removed and will remain in the body for many years. Third, each seed preferably includes a radiopaque marker so that it can be located at the treatment site with the aid of fluoroscopy. Fourth, the protective package and the radiopaque marker preferably do not cast "shadows" in the irradiation pattern of the isotope. Fifth, the isotope is preferably evenly distributed within the protective package so as to avoid any "hot spots" or "cold spots" of radiation. Sixth, the radiation energy or "activity" of the isotope must be carefully chosen so that the dosimetry or patient dosage of the seeds is predictable and controlled over the radioactive life of the seeds.
The state of the art of radioactive therapeutic seeds is substantially disclosed in seven U.S. patents: U.S. Pat. No. 5,713,828 to Coniglione for "Hollow-Tube Brachytherapy Device", U.S. Pat. No. 5,405,309 to Carden, Jr. for "X-Ray Emitting Interstitial Implants", U.S. Pat. No. 4,891,165 to Suthanthiran for "Device and Method for Encapsulating Radioactive Materials" and U.S. Pat. No. 4,784,116 to Russell, Jr. et al. for "Capsule for Interstitial Implants", U.S. Pat. No. 4,702,228 to Russell, Jr. et al. for "X-Ray Emitting Interstitial Implants", U.S. Pat. No. 4,323,055 to Kubiatowicz for "Radioactive Iodine Seed", and U.S. Pat. No. 3,351,049 to Lawrence for "Therapeutic Metal Seed Containing within a Radioactive Isotope Disposed on a Carrier and Method of Manufacture".
The Lawrence patent, which issued in 1967, describes many of the essential features of radioactive therapeutic seeds. Lawrence describes radioactive isotopes (I-125, Pd-103, Cs-131, Xe-133, and Yt-169) which emit low energy X-rays and which have relatively short half-lives. When implanted at a treatment site, these isotopes provide sufficient radiotherapy without posing a radiation danger to the medical practitioner(s), people in the vicinity of the patient, or other parts of the patient's body. Lawrence further describes a protective capsule which contains the isotope and prevents it from migrating throughout the body where it might interfere with healthy tissue. The capsule is cylindrical and made of low atomic number (low Z) biocompatible materials such as stainless steel or titanium which do not absorb X-rays. The isotope is coated on a rod shaped carrier made of similar X-ray transparent (or translucent) material and is placed inside the capsule cylinder. The ends of the capsule cylinder are closed by swaging or spinning and soldering or welding. According to a preferred embodiment, Lawrence places a radiopaque marker inside the seed. In one embodiment, the marker is a wire embedded inside the carrier rod. The wire is made of high atomic number (high Z) material such as gold or tungsten which absorb X-rays.
In 1980, Kubiatowicz made a minor improvement in the basic Lawrence design by providing that the entire isotope carrier be made of radiopaque material such as silver. Kubiatowicz recognized that since the isotope was carried on the entire outer surface of the carrier, there was no need to make the carrier body X-ray transparent as suggested by Lawrence. The larger radiopaque carrier body described by Kubiatowicz makes the seeds easier to see with X-ray or fluoroscopic examination. Thus, the seeds may be placed more accurately at/around the treatment site.
Several years later, Russell, Jr. et al., in U.S. Pat. Nos. 4,707,228 and 4,784,116, explained that the capsule design of Lawrence and Kubiatowicz produces anisotropic angular radiation distribution. According to Russell, Jr. et al., the shell forming techniques used in the Lawrence-type seeds results in large beads of shell material at the ends of the seeds. These beads substantially shield radiation thereby casting shadows in the irradiation pattern of the isotope. Russell, Jr. et al. proposed a new seed design to solve this problem. In particular, Russell, Jr. et al. proposed a seed having a cylindrical container which is sealed with end caps which have a wall thickness that is substantially the same as the wall thickness of the cylindrical container. The end caps are attached to the cylindrical container by welding or crimping.
An alternate solution to the non-uniform radiation pattern of the Lawrence-type seeds was proposed by Suthanthiran in U.S. Pat. No. 4,891,165. Suthanthiran's solution was to form a seed capsule from two interfitting sleeves, each having one open end and one closed end. The thickness of the sleeve side walls and their closed ends is such that when the sleeves are interfitted the total side wall thickness of the assembled capsule is approximately equal to the end wall thickness.
Other improvements in radioactive therapeutic seeds are disclosed in U.S. Pat. No. 5,405,309 which describes a safe isotopically pure Pd-103 seed, and U.S. Pat. No. 5,713,828 which describes a hollow tube seed which can be implanted with suture material.
Despite the fact that radioactive therapeutic seeds have been in use for over thirty years and despite the several significant improvements made in these seeds, many concerns still exist regarding their design and construction.
While significant attention has been given to the methods by which a cylindrical seed capsule is sealed, it is still difficult to seal the ends of such a small cylindrical capsule without adversely affecting the effective radiation dosimetry of the seed. Moreover, given the size of the seeds, it is very difficult to perform any welding of the capsules without adversely affecting the contents in some way.
While it is well known that the radioactive therapeutic seeds have virtually no shelf-life due to the relatively short half-life of the isotope used, little if any attention has been given to how this affects seed design and the manufacturing process. In particular, when seeds are manufactured from a stock pile of isotope, the first manufactured seeds will have a higher apparent radioactivity level than the last manufactured seeds. It is important for the practitioner to know the effective therapeutic dose contained in each seed and it is preferable that each seed have a predictable and consistent level of radioactivity.